The present invention relates generally to separation and sorting of carbon nanotubes according to desired properties, and more specifically to enriching the chirality of genomic single wall carbon nanotubes using DNA.
Single wall carbon nanotubes (SWNTs) have attracted significant attention because of their potential in a vast range of applications, including nanoscale circuits, conductors, electrochemical probes, transistors and photovoltaic devices. In order to achieve optimal performance of SWNTs for real-world electronic applications, the diameter, type and chirality have to be effectively sorted. Any process for sorting SWNTs has to be scalable, non-destructive and economical.
Separating nanotubes according to desired properties is still proving a challenging task, especially single-walled carbon nanotube sorting, because the composition and chemical properties of SWNTs of different types are very similar, making conventional separation techniques difficult and inefficient.
Prior art separation techniques for SWNTs rely on preferential electron transfer on metallic SWNTs treated with diazonium salts, dielectrophoresis, enhanced chemical affinity of semiconducting SWNTs with octadecylamine, and wrapping of SWNTs with single-stranded DNA. The selectivity of these methods can be further enhanced by vigorous centrifugation of prepared dispersions and the use of ion-exchange chromatography (IEX).
In 2003, Ming Zheng of DuPont and coworkers found that DNA strands could be used to separate CNTs according to their electronic characteristics. The discovery was reported in articles in Science (“Structure-Based Carbon Nanotube Sorting by Sequence-Dependent DNA Assembly”) and Nature (“DNA-assisted dispersion and separation of carbon nanotubes”) and cited later by Forbes magazine as one of the top five nanotechnology breakthroughs of 2003.
The DuPont team discovered an oligonucleotide sequence (that is, a short, single-stranded nucleic acid fragment, in this case a d(GT)20 oligomer DNA sequence) that self-assembles into a helical structure around individual nanotubes, creating DNA-CNT hybrids with electrostatic properties that depend on tube diameter and electrical properties. CNTs can be separated on the basis of these electrostatic properties using anion exchange chromatography. The separation of metallic and semiconducting nanotubes was improved compared with other techniques, and separation on the basis of tube diameter became possible.
The price for d(GT)20 is typically $25,000/gm and usually oligo-DNA assisted SWNT dispersion experiments are carried out with a DNA:SWNT weight ratio of 1:1, discarding the majority of unbound DNA. This poses a high price of $25,000 for oligo-DNA in treating every gm of carbon nanotubes, and thus, a cost-effective nucleic acid system is highly in demand.
It is seen, therefore, that there is a need for less expensive, as well as less complex, processes for separation and sorting of carbon nanotubes, particularly on the basis of chirality.